1. Field of the Invention
The present invention relates to an optical functional film in which stacked films are made of the same material and have an optical function, and a stress control has been applied to each film, and a method of forming the optical functional film. The present invention also relates to a spatial light modulator, a spatial light modulator array, an image forming device, and a flat panel display which use the optical functional film.
2. Description of the Art
A dielectric multilayer film in the related art is produced by alternately stacking high-refractive index films and low-refractive index films. As useful materials, for example, TiO2 is usually used in a high-refractive index film, and SiO2 is usually used in a low-refractive index film. In the case where such a dielectric multilayer film is to be produced as an optically reflective film, the materials are stacked in the sequence of high and low refractive indices so as to attain an optical length of λ/4 with respect to the wavelength of light. This production process has problems of curvature of a support member, a crack, and a film flake which are caused by an internal stress of the dielectric multilayer film.
In “Method of producing a dichroic mirror” of JP-A-6-186418 (paragraph Nos. [00009] to [0011]), a method of correcting curvature is disclosed. In a dichroic mirror, dielectric thin films for varying the light transmittance depending on the wavelength are stacked on a substrate. The dielectric thin films include low-refractive index films made of SiO2, and high-refractive index films made of TiO2, and are grown in a vacuum or an atmosphere of a specific gas by vacuum deposition. In the disclosed method, curvature which may be caused in the film growing process is compensated by performing the film growing process on a substrate having a shape that is opposite to the shape of the curvature.
In “Multilayer film structure and surface-emitting laser” of JP-A-8-307008 (paragraph Nos. [0022] to [0029], FIG. 2), a method of preventing distortion from occurring is disclosed. When a multilayer film 4 which is to be applied to a reflecting mirror of a surface-emitting laser is formed by a dielectric multilayer film of SiO2 films 2 and TiO2 films 3, a contraction stress which is produced in the SiO2 films and is indicated by the arrows is different in magnitude from an expansion stress which is produced in the TiO2 films as shown in FIG. 16B, so that contraction and expansion occur in the stacked planes, thereby causing distortion in a substrate. By contrast, in the disclosed method, as shown in FIG. 16A, a control is conducted while conditions such as the deposition rate and the deposition atmosphere are selected so that an expansion stress and a contraction stress have an equal magnitude. Namely, a contraction stress/tensile stress control is conducted so that an expansion stress and a contraction stress which are produced in respective alternately stacked films are made equal to each other to balance and cancel out. Therefore, the structure is improved so that distortion is not produced.
FIG. 17 shows an example of a Fabry-Perot filter which is disclosed in JP-A-2002-174721 (paragraph Nos. [0025] to [0035], FIG. 5), and in which a dielectric multilayer film is used. In the figure, shown is a three-layer structure consisting of a high-refractive index film F1 which serves as a movable mirror, and which exhibits a compression stress, a low-refractive index film F2 which exhibits a tensile stress, and a high-refractive index film F3 which exhibits a compression stress. The multilayer optical thin film has an optical film thickness of λ/4, or is a multilayer film corresponding to a single-layer film having an optical film thickness of λ/4.
When a movable mirror is formed by a multilayer optical thin film in which films (tensile stress films) exhibiting a tensile-stress and films (compression stress films) exhibiting a compression stress are stacked, or by that in which tensile stress films exhibiting different tensile stresses are stacked, a self-standing movable mirror can be formed so as to be upward convex and downward convex. The tensile stress films and the compression stress films can be formed respectively by, for example, polysilicon (single crystal silicon), silicon oxide, silicon nitride. Combinations of a tensile stress film and a compression stress film, and those of tensile stress films include combinations of: a compression stress film (polysilicon) and a tensile stress film (silicon nitride); a compression stress film (polysilicon) and a tensile stress film (silicon oxide); a compression stress film (silicon oxide) and a tensile stress film (polysilicon); a compression stress film (silicon nitride) and a tensile stress film (polysilicon); a tensile stress film (polysilicon) and a tensile stress film (silicon nitride); and a tensile stress film (polysilicon) and a tensile stress film (silicon oxide).
When the multilayer optical thin film (movable mirror) is configured as an optical thin film of a multilayer structure in which the high-refractive index film F1, the low-refractive index film F2, and the high-refractive index film F3 are stacked in this sequence, it is possible to enhance the degree of freedom in design of the film stress.
However, all the above-described techniques in the related art have a serious problem in that the adhesiveness between films remains to be inferior.